Hexameric structure and assembly of the interleukin-6/IL-6 alpha-receptor/gp130 complex

Large-scale preparation of biologically active mouse and rat leptins and their L39A/D40A/F41A muteins which act as potent antagonists

Expression plasmids encoding mouse and rat leptins and their L39A/D40A/F41A muteins were prepared. The proteins were expressed in Escherichia coli, refolded and purified to homogeneity, yielding electrophoretically pure, over 98% monomeric protein. Circular dichroism (CD) analysis revealed that the mutations hardly affect the leptins' secondary structure, and they were similar to previously reported CD spectra for human leptin. Both mouse and rat leptins were biologically active in promoting proliferation in BAF/3 cells stably transfected with the long form of human leptin receptor. The mutations did not change the binding properties to BAF/3 cells as compared, respectively, to non-mutated mouse, rat or human leptins, or their ability to form 1:1 complexes with the leptin-binding domain of chicken leptin receptor. In contrast, their biological activity, tested in a BAF/3 proliferation assay, was abolished and both became potent antagonists. As the LDF (amino acids 39-41) sequence is preserved in all known leptins, the present results substantiate the hypothesis that this sequence plays a pivotal role in leptins' site III and that interaction of leptin with its receptors resembles the corresponding interactions of interleukin-6 and granulocyte colony-stimulating factor their receptors.

The rational designed antagonist derived from the complex structure of interleukin-6 and its receptor affectively blocking interleukin-6 might be a promising treatment in multiple myeloma

Human interleukin-6 is involved in the maintenance and progression of several diseases such as multiple myeloma (MM), rheumatoid arthritis, or osteoporosis. Our previous work demonstrated that an interleukin-6 antagonist peptide (named PT) possessed potential bioactivity to antagonize the function of hIL-6 and could efficiently induce the growth arrest and apoptosis of XG-7 and M1 cells in a dose-dependent manner. In this study, the theoretical interaction of the peptide PT with its receptor was analyzed further more with molecular docking and molecular dynamics methods. The theoretical studies showed that PT possessed very high affinity to interleukin-6R and offered a practical means of imposing long-term blockade of interleukin-6 activity in vivo. According to the theoretical results, the biological evaluation of PT was researched on two different cells models with more sensitive approaches: (1) The antagonist activity of PT was studied on the interleukin-6 dependent MM cells (XG-7) cultured with interleukin-6. In the other interleukin-6 dependent MM cells (SKO-007), they survived themselves by auto/paracrine without the exogenous interleukin-6, and also could be antagonized by PT. The therapeutic value of PT only limited on the interleukin-6 dependent category in MM. (2) Myeloid leukemia M1 cells were induced for growth arrest and apoptosis in response to interleukin-6. The results supported our previous findings and showed that PT could be evaluated by protecting the cells from interleukin-6 induced apoptosis. In conclusion, PT could induce interleukin-6-dependent XG-7 and SKO-007 cells to apoptosis while inhibit interleukin-6-stimulated apoptosis in M1 cells.

Mapping of binding site III in the leptin receptor and modeling of a hexameric leptin.leptin receptor complex

The leptin.leptin receptor (LR) system shows strong similarities to the long chain cytokine interleukin-6 (IL-6) and granulocyte colony-stimulating factor (G-CSF) cytokine.cytokine receptor systems. The IL-6 family cytokines interact with their receptors through three different binding sites (I-III). We demonstrated previously that leptin has similar binding sites I-III and mapped the interactions between binding site II and cytokine receptor homology domain II (CRH2) (Peelman, F., Van Beneden, K., Zabeau, L., Iserentant, H., Ulrichts, P., Defeau, D., Verhee, A., Catteeuw, D., Elewaut, D., and Tavernier, J. (2004) J. Biol. Chem. 279, 41038-41046). In this study, we built homology models for the CRH1 and Ig-like domains of the LR. The Ig-like domain shows a large conserved surface patch in the beta-sheet formed by beta-strands 3, 6, and 7. Mutations in this patch almost completely abolished the leptin-induced STAT3-dependent reporter activity. We propose that a conserved cluster of residues Leu370, Ala407, Tyr409, His417, and His418 forms the center of binding site III of the LR. We built a hexameric leptin.LR complex model based on the hexameric IL-6 complex. In this model, a conserved hydrophobic protuberance of Val36, Thr37, Phe41, and Phe43 in the A-B loop of leptin fits perfectly in the CRH2 domain, corresponding to the IL-6 alpha-receptor, and forms the center of binding site I. The 2:4 hexameric leptin.LR complex offers a rational explanation for mutagenesis studies and residue conservation.

Selective protein-protein interactions driven by a phenylalanine interface

Highly specific protein-protein interfaces have been the subject of considerable study for their potential utility in disrupting or interrogating cellular signaling and control networks. We report that coiled-coil sequences decorated with phenylalanine core residues fold into stable alpha-helical bundles and that these self-sort from similar peptide assemblies with aliphatic core side chains. For self-assembled ensembles derived from 30-residue monomeric peptides, the DeltaG of specificity is -1.5 kcal/mol, comparable with earlier self-sorting coiled-coil systems. Intriguingly, although this interface is constructed from canonical amino acids, it does not appear to have been exploited in native proteins.

Homodimeric cross-over structure of the human granulocyte colony-stimulating factor (GCSF) receptor signaling complex

A crystal structure of the signaling complex between human granulocyte colony-stimulating factor (GCSF) and a ligand binding region of GCSF receptor (GCSF-R), has been determined to 2.8 A resolution. The GCSF:GCSF-R complex formed a 2:2 stoichiometry by means of a cross-over interaction between the Ig-like domains of GCSF-R and GCSF. The conformation of the complex is quite different from that between human GCSF and the cytokine receptor homologous domain of mouse GCSF-R, but similar to that of the IL-6/gp130 signaling complex. The Ig-like domain cross-over structure necessary for GCSF-R activation is consistent with previously reported thermodynamic and mutational analyses.

IL-3, IL-5, and GM-CSF signaling: crystal structure of the human beta-common receptor

The cytokines, interleukin-3 (IL-3), interleukin-5 (IL-5), and granulocyte-macrophage colony stimulating factor (GM-CSF), are polypeptide growth factors that exhibit overlapping activities in the regulation of hematopoietic cells. They appear to be primarily involved in inducible hematopoiesis in response to infections and are involved in the pathogenesis of allergic and inflammatory diseases and possibly in leukemia. The X-ray structure of the beta common (betac) receptor ectodomain has given new insights into the structural biology of signaling by IL-3, IL-5, and GM-CSF. This receptor is shared between the three ligands and functions together with three ligand-specific alpha-subunits. The structure shows betac is an intertwined homodimer in which each chain contains four domains with approximate fibronectin type-III topology. The two betac-subunits that compose the homodimer are interlocked by virtue of the swapping of beta-strands between domain 1 of one subunit and domain 3 of the other subunit. Site-directed mutagenesis has shown that the interface between domains 1 and 4 in this unique structure forms the functional epitope. This epitope is similar to those of other members of the cytokine class I receptor family but is novel in that it is formed by two different receptor chains. The chapter also reviews knowledge on the closely related mouse beta(IL-3) receptor and on the alpha-subunit-ligand interactions. The knowledge on the two beta receptors is placed in context with advances in understanding of the structural biology of other members of the cytokine class I receptor family.

Characterization and expression analysis of a chicken interleukin-6 receptor alpha

Interleukin-6 (IL-6) is a multifunctional cytokine that plays roles in regulating immune responses, acute phase reactions and hematopoiesis. IL-6 signaling is regulated by two receptors, a specific alpha chain (IL-6Ralpha) and a signal transducer, gp130. In this study, cDNA encoding the 445 amino acid propeptide of chicken IL-6Ralpha (chIL-6Ralpha) was identified. The predicted 445 amino acids showed approximately 40% sequence identity with mammalian homologues. In a domain search, chIL-6Ralpha had a signal peptide of 20 residues, an immunoglobulin-like (IG) domain of 71 residues and a fibronectin-type III (FN III) domain of 85 residues. On comparison with mammalian homologues, four conserved cysteine residues and the WSXWS motif were observed in the N- and C-terminal regions of the FN III domain, respectively. Expression analysis revealed that chIL-6Ralpha is strongly expressed in liver and the chicken hepatoma cell line LMH. These findings indicate that the identified chicken cDNA sequence encodes a chIL-6Ralpha homologue.

The structure of the interleukin-15 alpha receptor and its implications for ligand binding

Interleukin (IL)-15 is a member of the small four alpha-helix bundle family of cytokines. IL-15 was discovered by its ability to mimic IL-2-mediated T-cell proliferation. Both cytokines share the beta and gamma receptor chains of the IL-2 receptor for signal transduction. However, in addition, they target specific alpha chain receptors IL-15Ralpha and IL-2Ralpha, respectively. The exceptionally high affinity binding of IL-15 to IL-15Ralpha is mediated by its sushi domain. Here we present the solution structure of the IL-15Ralpha sushi domain solved by NMR spectroscopy and a model of its complex with IL-15. The model shows that, rather than the familiar hydrophobic forces dominating the interaction interface between cytokines and their cognate receptors, the interaction between the IL-15 and IL-15Ralpha complex involves a large network of ionic interactions. This type of interaction explains the exceptionally high affinity of the IL-15.IL-15Ralpha complex, which is essential for the biological effects of this important cytokine and which is not observed in other cytokine/cytokine receptor complexes.

Pathological role of IL-6 in the experimental allergic bronchial asthma in mice

Although allergic asthma was described to be associated with the presence of mucosal T helper (Th)2 cells, it is not entirely clear which factors are responsible for priming of T cells to differentiate into Th2 effector cells in this disease. Interleukin (IL)-6 has been recognized as important because it is secreted by cells of the innate immunity and induces the expansion of the Th2 effector cells, which are major players of the adaptive immune responses. Additionally, IL-6 released by dendritic cells (DCs) inhibits the suppressive function of CD4+CD25+ T regulatory cells, thus inhibiting the peripheral tolerance. The signal transduction of IL-6 has recently taught us how this cytokine influences different aspects of the immune response, especially under pathological conditions. IL-6 can bind to the soluble IL-6R, increased after allergen challenge in asthmatic patients, and, through a mechanism called trans-signaling, induces proliferation of cells expressing the cognate receptor gp130. This mechanism appears to be used for proliferation by developed Th2 cells in the airways. In contrast, through the membrane-bound IL-6R, IL-6 controls CD4+CD25+ survival, as well as the initial stages of the Th2 cells development in the lung. These findings impact the establishment of new therapies for allergic diseases; indeed, blockade of the soluble IL-6R through the fusion protein gp130Fc reduces Th2 cells in the lung, and by blocking the membrane-bound IL-6R, anti-IL-6R antibody treatment induces the number of T-regulatory cells in the lung, thereby reducing the local number of CD4+ T-effector cells in experimental asthma.

Dimerization of the cytokine receptors gp130 and LIFR analysed in single cells

The cytokine receptor gp130 is the shared signalling subunit of the IL-6-type cytokines. Interleukin-6 (IL-6) signals through gp130 homodimers whereas leukaemia inhibitory factor (LIF) exerts its action through a heterodimer of gp130 and the LIF receptor (LIFR). Related haematopoietic receptors such as the erythropoietin receptor have been described as preformed dimers in the plasma membrane. Here we investigated gp130 homodimerization and heterodimerization with the LIFR by fluorescence resonance energy transfer (FRET) and bimolecular fluorescence complementation (BiFC). We detected a FRET signal between YFP- and CFP-tagged gp130 at the plasma membrane of unstimulated cells that does not increase upon IL-6 stimulation. However, FRET between YFP-tagged gp130 and CFP-tagged LIFR considerably increased upon LIF stimulation. Using a BiFC approach that detects stable interactions we show that fluorescence complementation of gp130 constructs tagged with matching 'halves' of fluorescent proteins increases upon IL-6 stimulation. Taken together, these findings suggest that transient gp130 homodimers on the plasma membrane are stabilized by IL-6 whereas heterodimerization of gp130 with the LIFR is mainly triggered by the ligand. This view is supported by the observation that the simultaneous action of two IL-6 binding domains on two gp130 molecules is required to efficiently recruit a fluorescent IL-6 (YFP-IL-6) to the plasma membrane.

Interleukin-6 and diabetes: the good, the bad, or the indifferent?

Inflammatory mechanisms play a key role in the pathogenesis of type 1 diabetes. Individuals who progress to type 2 diabetes display features of low-grade inflammation years in advance of disease onset. This low-grade inflammation has been proposed to be involved in the pathogenetic processes causing type 2 diabetes. Mediators of inflammation such as tumor necrosis factor-alpha, interleukin (IL)-1beta, the IL-6 family of cytokines, IL-18, and certain chemokines have been proposed to be involved in the events causing both forms of diabetes. IL-6 has in addition to its immunoregulatory actions been proposed to affect glucose homeostasis and metabolism directly and indirectly by action on skeletal muscle cells, adipocytes, hepatocytes, pancreatic beta-cells, and neuroendocrine cells. Here we argue that IL-6 action-in part regulated by variance in the IL-6 and IL-6alpha receptor genes-contributes to, but is probably neither necessary nor sufficient for, the development of both type 1 and type 2 diabetes. Thus, the two types of diabetes are also in this respect less apart than apparent. However, the mechanisms are not clear, and we therefore propose future directions for studies in this field.

Structure of the quaternary complex of interleukin-2 with its alpha, beta, and gammac receptors

Interleukin-2 (IL-2) is an immunoregulatory cytokine that acts through a quaternary receptor signaling complex containing alpha (IL-2Ralpha), beta (IL-2Rbeta), and common gamma chain (gc) receptors. In the structure of the quaternary ectodomain complex as visualized at a resolution of 2.3 angstroms, the binding of IL-2Ralpha to IL-2 stabilizes a secondary binding site for presentation to IL-2Rbeta. gammac is then recruited to the composite surface formed by the IL-2/IL-2Rbeta complex. Consistent with its role as a shared receptor for IL-4, IL-7, IL-9, IL-15, and IL-21, gammac forms degenerate contacts with IL-2. The structure of gammac provides a rationale for loss-of-function mutations found in patients with X-linked severe combined immunodeficiency diseases (X-SCID). This complex structure provides a framework for other gammac-dependent cytokine-receptor interactions and for the engineering of improved IL-2 therapeutics.

Cytokine recognition by human interleukin 5 receptor

The activation of interleukin 5 (IL-5) receptor is a dynamic process that depends on specific interaction of IL-5 with IL-5 receptor alpha, the formation of oligomeric receptor complexes with receptor beta, and the initiation of cytoplasmic phosphorylation events. These steps culminate in the triggering of a cellular response. Important advances have been made recently in understanding the molecular mechanisms of cytokine recognition, receptor assembly, and signal triggering. Cytokine recognition can be envisioned by relating structure to function in IL-5 and IL-5 receptor alpha. A pair of charge-complementary regions plays an essential role in the specific interaction between IL-5 receptor alpha and IL-5. Moreover, peptide library methodology has led to the discovery of IL-5 receptor alpha antagonists that mimic key elements in IL-5 receptor recognition. Because IL-5 has been implicated in the pathology of eosinophil-related inflammatory diseases, revealing the key recognition elements of IL-5, IL-5 mimetic peptides, and IL-5 receptor alpha could help drive the design of new compounds for therapeutic treatment against allergic inflammatory diseases such as asthma.

Bacterial induction of TNF-alpha converting enzyme expression and IL-6 receptor alpha shedding regulates airway inflammatory signaling

Airway epithelial cells have a major role in initiating inflammation in response to bacterial pathogens. Through the immediate induction of CXCL8 and cytokine expression, polymorphonuclear cells are mobilized and activated to eradicate the infecting organisms. However, the influx of polymorphonuclear cells and the effects of their toxic exoproducts impede respiratory function. We postulated that respiratory epithelial cells must also participate in the regulation of their own proinflammatory signaling. Both Staphylococcus aureus and Pseudomonas aeruginosa were found to potently activate IL-6 expression immediately upon contact with epithelial cells, and by 1 h induced TNF-alpha converting enzyme (TACE) transcription. By 4 h of bacterial exposure, TACE colocalized with IL-6Ralpha on the apical surface of airway cells, and by 24 h, soluble IL-6Ralpha accumulated in the cell culture supernatant. Epithelial IL-6 and soluble IL-6Ralpha were shown to participate in trans-signaling, interacting with membrane-associated gp130 to activate CCL-2 expression and inhibit additional CXCL8 production. Thus, bacteria are physiological activators of TACE expression, which provides a mechanism to regulate inflammatory signaling that is initiated by airway epithelial cells.

Identification of the hydrophobic strand in the A-B loop of leptin as major binding site III: implications for large-scale preparation of potent recombinant human and ovine leptin antagonists

Interaction of leptin with its receptors resembles that of interleukin-6 and granulocyte colony-stimulating factor, which interact with their receptors through binding sites I-III. Site III plays a pivotal role in receptors' dimerization or tetramerization and subsequent activation. Leptin's site III also mediates the formation of an active multimeric complex through its interaction with the IGD (immunoglobulin-like domain) of LEPRs (leptin receptors). Using a sensitive hydrophobic cluster analysis of leptin's and LEPR's sequences, we identified hydrophobic stretches in leptin's A-B loop (amino acids 39-42) and in the N-terminal end of LEPR's IGD (amino acids 325-328) that are predicted to participate in site III and to interact with each other in a beta-sheet-like configuration. To verify this hypothesis, we prepared and purified to homogeneity (as verified by SDS/PAGE, gel filtration and reverse-phase chromatography) several alanine muteins of amino acids 39-42 in human and ovine leptins. CD analyses revealed that those mutations hardly affect the secondary structure. All muteins acted as true antagonists, i.e. they bound LEPR with an affinity similar to the wild-type hormone, had no agonistic activity and specifically inhibited leptin action in several leptin-responsive in vitro bioassays. Alanine mutagenesis of LEPR's IGD (amino acids 325-328) drastically reduced its biological but not binding activity, indicating the importance of this region for interaction with leptin's site III. FRET (fluorescence resonance energy transfer) microscopy experiments have documented that the transient FRET signalling occurring upon exposure to leptin results not from binding of the ligand, but from ligand-induced oligomerization of LEPRs mediated by leptin's site III.

The N-terminal cytokine binding domain of LIFR is required for CNTF binding and signaling

Ciliary neurotrophic factor (CNTF) forms a functional receptor complex containing the CNTF receptor, gp130, and the leukemia inhibitory factor receptor (LIFR). However, the nature and stoichiometry of the receptor-mediated interactions in this complex have not yet been fully resolved. We show here that signaling by CNTF, but not by LIF or oncostatin M (OSM), was abolished in cells overexpressing a LIFR mutant with the N-terminal cytokine binding domain deleted. Our results illustrate molecular differences between the CNTF active receptor complex and those of LIF and OSM and provide further support for the hexameric model of the CNTF receptor complex.

Crystallization of a 2:2 complex of granulocyte-colony stimulating factor (GCSF) with the ligand-binding region of the GCSF receptor

The granulocyte-colony stimulating factor (GCSF) receptor receives signals for regulating the maturation, proliferation and differentiation of the precursor cells of neutrophilic granulocytes. The signalling complex composed of two GCSFs (GCSF, 19 kDa) and two GCSF receptors (GCSFR, 34 kDa) consisting of an Ig-like domain and a cytokine-receptor homologous (CRH) domain was crystallized. A crystal of the complex was grown in 1.0 M sodium formate and 0.1 M sodium acetate pH 4.6 and belongs to space group P4(1)2(1)2 (or its enantiomorph P4(3)2(1)2), with unit-cell parameters a = b = 110.1, c = 331.8 A. Unfortunately, this crystal form did not diffract beyond 5 A resolution. Since the heterogeneity of GCSF receptor appeared to prevent the growth of good-quality crystals, the GCSF receptor was fractionated by anion-exchange chromatography. Crystals of the GCSF-fractionated GCSF receptor complex were grown as a new crystal form in 0.2 M ammonium phosphate. This new crystal form diffracted to beyond 3.0 A resolution and belonged to space group P3(1)21 (or its enantiomorph P3(2)21), with unit-cell parameters a = b = 134.8, c = 105.7 A.

Reversal of some viral IL-6 electrostatic properties compared to IL-6 contributes to a loss of alpha receptor component recruitment

Human interleukin-6 (hIL-6) is a pleiotropic mediator of activation and proliferation across a large number of different cell types. Human herpesvirus-8 (HHV-8) has been associated with classical and AIDS-related Kaposi's sarcoma (KS). HHV-8 encodes viral IL-6 (vIL-6), a functional homolog of human interleukin-6, that promotes the growth of KS and of some lymphoma cells. Signaling induced by human IL-6 requires recruitment of the glycoprotein gp130, which acts as the signal transducing chain, and of IL-6Ralpha, which is necessary for cognate recognition and high affinity receptor complex formation. In contrast, the formation of a functional complex between vIL-6 and gp130 does not require the presence of IL-6Ralpha. The physico-chemical properties of vIL-6 have been analyzed and compared to those of hIL-6 and of the receptor chains, gp130 and IL-6Ralpha. Interaction sites on vIL-6 involve more hydrophobic residues than those of hIL-6. The electrostatic fields induced by vIL-6 and IL-6Ralpha are repulsive and prevent interaction between vIL-6 and IL-6Ralpha, whereas the electrostatic field induced by hIL-6 steers the complex formation with IL-6Ralpha. Subsequently, electrostatic binding free energy in the vIL-6/IL-6Ralpha complex is destabilizing, whereas it is stabilizing in the complex comprising hIL-6. These properties result from charge reversals between viral and human IL-6, an unusual phenomenon of amino acid substitutions within a homologous protein family. This suggests a selection pressure for vIL-6 to by-pass the IL-6Ralpha control of host defense against virus infection. This selection pressure has yielded the reversal of electrostatic properties of vIL-6 when compared to hIL-6.

A model of the ternary complex of interleukin-10 with its soluble receptors

BACKGROUND

Interleukin-10 (IL-10) is a cytokine whose main biological function is to suppress the immune response by induction of a signal(s) leading to inhibition of synthesis of a number of cytokines and their cellular receptors. Signal transduction is initiated upon formation of a ternary complex of IL-10 with two of its receptor chains, IL-10R1 and IL-10R2, expressed on the cell membrane. The affinity of IL-10R1 toward IL-10 is very high, which allowed determination of the crystal structure of IL-10 complexed with the extracellular/soluble domain of IL-10R1, while the affinity of IL-10R2 toward either IL-10 or IL-10/sIL-10R1 complex is quite low. This so far has prevented any attempts to obtain structural information about the ternary complex of IL-10 with its receptor chains.

RESULTS

Structures of the second soluble receptor chain of interleukin-10 (sIL-10R2) and the ternary complex of IL-10/sIL-10R1/sIL-10R2 have been generated by homology modeling, which allowed us to identify residues involved in ligand-receptor and receptor-receptor interactions.

CONCLUSION

The previously experimentally determined structure of the intermediate/binary complex IL-10/sIL-10R1 is the same in the ternary complex. There are two binding sites for the second receptor chain on the surface of the IL-10/sIL-10R1 complex, involving both IL-10 and sIL-10R1. Most of the interactions are hydrophilic in nature, although each interface includes two internal hydrophobic clusters. The distance between C-termini of the receptor chains is 25 A, which is common for known structures of ternary complexes of other cytokines. The structure is likely to represent the biologically active signaling complex of IL-10 with its receptor on the surface of the cell membrane.

Dynamics of the gp130 cytokine complex: a model for assembly on the cellular membrane

Cytokines of the interleukin-6 (IL-6)-type family all bind to the glycoprotein gp130 on the cell surface and require interaction with two gp130 or one gp130 and another related signal transducing receptor subunit. In addition, some cytokines of this family, such as IL-6, interleukin-11, ciliary neurotrophic factor, neuropoietin, cardiotrophin-1, and cardiotrophin-1-like-cytokine, interact with specific ligand binding receptor proteins. High- and low-affinity binding sites have been determined for these cytokines. So far, however, the stoichiometry of the signaling receptor complexes has remained unclear, because the formation of the cytokine/cytokine-receptor complexes has been analyzed with soluble receptor components in solution, which do not necessarily reflect the situation on the cellular membrane. Consequently, the binding affinities measured in solution have been orders of magnitude below the values obtained with whole cells. We have expressed two gp130 extracellular domains in the context of a Fc-fusion protein, which fixes the receptors within one dimension and thereby restricts the flexibility of the proteins in a fashion similar to that within the plasma membrane. We measured binding of IL-6 and interleukin-b receptor (IL-6R) by means of fluorescence-correlation spectroscopy. For the first time we have succeeded in recapitulating in a cell-free condition the binding affinities and dynamics of IL-6 and IL-6R to the gp130 receptor proteins, which have been determined on whole cells. Our results demonstrate that a dimer of gp130 first binds one IL-6/IL-6R complex and only at higher ligand concentrations does it bind a second IL-6/IL-6R complex. This view contrasts with the current perception of IL-6 receptor activation and reveals an alternative receptor activation mechanism.

The structure of interleukin-2 complexed with its alpha receptor

Interleukin-2 (IL-2) is an immunoregulatory cytokine that binds sequentially to the alpha (IL-2Ralpha), beta (IL-2Rbeta), and common gamma chain (gammac) receptor subunits. Here we present the 2.8 angstrom crystal structure of a complex between human IL-2 and IL-2Ralpha, which interact in a docking mode distinct from that of other cytokine receptor complexes. IL-2Ralpha is composed of strand-swapped "sushi-like" domains, unlike the classical cytokine receptor fold. As a result of this domain swap, IL-2Ralpha uses a composite surface to dock into a groove on IL-2 that also serves as a binding site for antagonist drugs. With this complex, we now have representative structures for each class of hematopoietic cytokine receptor-docking modules.

Biological activity of recombinant chicken interleukin-6 in chicken hybridoma cells

Interleukin-6 (IL-6), a multipotential cytokine that plays roles in regulating immune responses, acute phase reactions and hematopoiesis, induces proliferation and antibody production in hybridoma cells. The biological activities of the recombinant chicken IL-6 (rchIL-6) were determined using murine and chicken hybridoma cells. Cell proliferation and tyrosine phosphorylation of signal transducer and activator of transcription-3 (STAT3) were induced by rchIL-6 in the IL-6-dependent murine hybridoma cell line MH60, whereas the recombinant protein exhibited no significant cell proliferation activity in chicken hybridoma cells but induced antibody production and tyrosine phosphorylation of STAT3. The lack of cell proliferation induced by rchIL-6 in HUC2-13 cells may have been because the cell line was not IL-6-dependent in contrast to MH60 cells. These results suggest that rchIL-6 may be useful for promoting antibody production of chicken hybridoma cells as well as for creating chicken hybridomas by cell fusion.

Signaling conformations of the tall cytokine receptor gp130 when in complex with IL-6 and IL-6 receptor

gp130 is a shared cytokine signaling receptor and the founding member of the 'tall' class of cytokine receptors. A crystal structure of the ligand-binding domains of gp130 in complex with human interleukin-6 (IL-6) and its a-receptor (IL-6Ralpha) revealed a hexameric architecture in which the gp130 membrane-distal regions were approximately 100 A apart, in contrast to the close apposition seen between short cytokine receptor complexes. Here we used single-particle EM to visualize the entire extracellular hexameric IL-6-IL-6Ralpha-gp130 complex, containing all six gp130 domains. The structure reveals that gp130 is bent such that the membrane-proximal domains of gp130 are close together at the cell surface, enabling activation of intracellular signaling. Variation in the receptor bend angles suggests a possible conformational transition from open to closed states upon ligand binding; this transition is probably representative of the other tall cytokine receptors.

Characterization of the interaction between interleukin-13 and interleukin-13 receptors

Interleukin-13 (IL-13) possesses two types of receptor: the heterodimer, composed of the IL-13Ralpha1 chain (IL-13Ralpha1) and the IL-4Ralpha chain (IL-4Ralpha), transducing the IL-13 signals; and the IL-13Ralpha2 chain (IL-13Ralpha2), acting as a nonsignaling "decoy" receptor. Extracellular portions of both IL-13Ralpha1 and IL-13Ralpha2 are composed of three fibronectin type III domains, D1, D2, and D3, of which the last two comprise the cytokine receptor homology modules (CRHs), a common structure of the class I cytokine receptor superfamily. Thus far, there has been no information about the critical amino acids of the CRHs or the role of the D1 domains of IL-13Ralpha1 and IL-13Ralpha2 in binding to IL-13. In this study, we first built the homology modeling of the IL-13.hIL-13 receptor complexes and then predicted the amino acids involved in binding to IL-13. By incorporating mutations into these amino acids, we identified Tyr-207, Asp-271, Tyr-315, and Asp-318 in the CRH of human IL-13Ralpha2, and Leu-319 and Tyr-321 in the CRH of human IL-13Ralpha1, as critical residues for binding to IL-13. Tyr-315 in IL-13Ralpha2 and Leu-319 in IL-13Ralpha1 are positionally conserved hydrophobic amino acid residues. Furthermore, by using D1 domain-deleted mutants, we found that the D1 domain is needed for the expression of IL-13Ralpha2, but not IL-13Ralpha1, and that the D1 domain of IL-13Ralpha1 is important for binding to IL-13, but not to IL-4. These results provide the basis for a precise understanding of the interaction between IL-13 and its receptors.

Structure-based design and characterization of a Novel IL-6 antagonist peptide

The development of rational methods to design antagonist peptides based on the 3-D structure of protein active region has, to now, been only marginally successful. This has been largely due to the difficulty of constraining the recognition elements of a mimetic structure to the relative conformational and spatial orientations present in the parent molecule. According to the 3-D complex structure of human interleukin-6 (hIL-6) and its receptor (hIL-6R), a novel antagonist peptide (named PT), which possessed potential bioactivity of hIL-6, was designed by the means of distance geometry, molecular modeling and molecular dynamics trajectory analysis. The bioactivity of the designed peptide (i.e. PT) was evaluated using XG-7 cells, a hIL-6-dependent B-cell line. PT possessed potential bioactivity to antagonize the function of hIL-6 and could efficiently induce the growth arrest and apoptosis of XG-7 cells in a dose-dependent manner.

Leptin receptor activation depends on critical cysteine residues in its fibronectin type III subdomains

The leptin receptor (LR) complex is composed of a single subunit belonging to the class I cytokine receptor family and exists as a preformed complex. The extracellular portion contains two cytokine receptor homology (CRH) domains, separated by an Ig-like domain and followed by two membrane-proximal fibronectin type III (FNIII) domains. The mechanisms underlying ligand-induced receptor activation are still poorly understood. LRs can exist as disulfide-linked dimers at the cell surface, even in the absence of leptin. We evaluated the role of the two unpaired cysteine residues (Cys-672 and Cys-751) in the FNIII domains in receptor clustering, leptin binding, and biological activity. Although mutation of cysteine on position 751 to serine has hardly any effect on ligand binding and receptor activation, the C672S mutant exhibits a marked reduction in STAT3-dependent signaling. The double mutant was completely devoid of biological activity, although leptin binding remained unaffected. Mutation of both residues resulted in complete loss of disulfide bridge formation of FNIII domains in solution. In contrast, no difference was observed in ligand-independent oligomerization of the membrane-bound receptor, suggesting a role for cysteines in the CRH2 domain in formation of the preformed LR complex. We propose a model wherein leptin-induced clustering of two preformed dimers forms the activated LR complex. Disulfide bridge formation involving Cys-672 and Cys-751 may be necessary for JAK activation and hence signaling.

Organ-specific regulation of pro-inflammatory molecules in heart, lung, and kidney following brain death

BACKGROUND

Nonspecific inflammatory events following brain death may increase the intensity of the immunological host response. The present study investigated the course of pro-inflammatory molecules in heart, lung, kidney, and plasma after brain death induction.

MATERIALS AND METHODS

Brain death was induced in five pigs by inflation of an intracranial Foley catheter and five pigs were sham-operated as controls. Each experiment was terminated 6 h after brain death/sham operation and the organs were harvested. We measured the mRNA and protein levels for TNF-alpha, IL-1beta, and IL-6 in heart, lung, kidney, and plasma. Additionally, the mRNA expression for IL-6R, ICAM-1, MCP-1, and TGF-beta was determined in each organ.

RESULTS

After 6 h, the plasma cytokine levels were higher in the brain-dead animals than in the sham-operated. In heart, lung, and kidney there was an increase in IL-6 and IL-1beta following brain death, while TNF-alpha was up-regulated in lung only (P < 0.05). MCP-1 and TGF-beta were significantly higher in heart and lung and IL-6R increased in heart after brain death (P < 0.05).

CONCLUSIONS

Brain death was associated with non-uniform cytokine expression patterns in the investigated organs. These expression patterns may cause variable pro-inflammatory priming resulting in different degrees of damage and explain the organ-specific variation in outcomes after transplantations.

Interactions underlying assembly of the Escherichia coli AcrAB-TolC multidrug efflux system

The major Escherichia coli multidrug efflux pump AcrAB-TolC expels a wide range of antibacterial agents. Using in vivo cross-linking, we show for the first time that the antiporter AcrB and the adaptor AcrA, which form a translocase in the inner membrane, interact with the outer membrane TolC exit duct to form a contiguous proteinaceous complex spanning the bacterial cell envelope. Assembly of the pump appeared to be constitutive, occurring in the presence and absence of drug efflux substrate. This contrasts with substrate-induced assembly of the closely related TolC-dependent protein export machinery, possibly reflecting different assembly dynamics and degrees of substrate responsiveness in the two systems. TolC could be cross-linked independently to AcrB, showing that their large periplasmic domains are in close proximity. However, isothermal titration calorimetry detected no interaction between the purified AcrB and TolC proteins, suggesting that the adaptor protein is required for their stable association in vivo. Confirming this view, AcrA could be cross-linked independently to AcrB and TolC in vivo, and calorimetry demonstrated energetically favourable interactions of AcrA with both AcrB and TolC proteins. AcrB was bound by a polypeptide spanning the C-terminal half of AcrA, but binding to TolC required interaction of N- and C-terminal polypeptides spanning the lipoyl-like domains predicted to present the intervening coiled-coil to the periplasmic coils of TolC. These in vivo and in vitro analyses establish the central role of the AcrA adaptor in drug-independent assembly of the tripartite drug efflux pump, specifically in coupling the inner membrane transporter and the outer membrane exit duct.

Variations of the interleukin-6 promoter are associated with features of the metabolic syndrome in Caucasian Danes

AIMS/HYPOTHESIS

The cytokine interleukin 6 (IL-6) is an essential regulator of the acute phase response associated with insulin-resistant states including type 2 diabetes and obesity. Three polymorphisms at positions -597, -572, and -174 of the IL6 promoter have been reported to influence IL6 transcription. The aim of this study was to investigate whether the IL6 promoter polymorphisms were associated with features of the WHO-defined metabolic syndrome and related quantitative traits in 7,553 Caucasian Danes.

METHODS

Using analysis of PCR-generated primer extension products by mass spectrometry we examined -597 G/A, -572 G/C, and -174 G/C IL6 variants in the population-based Inter99 study cohort of middle-aged people (n=6,164) and in a group of type 2 diabetic patients (n=1,389).

RESULTS

The -174 G/C and -597 G/A polymorphisms were in strong linkage disequilibrium (R(2)=0.95). In the Inter99 cohort the -174 G-allele was associated with insulin resistance (p<0.02) and dyslipidaemia (p<0.007) whereas the C-allele of the -572 polymorphism was associated with increased serum insulin release during an OGTT (p<0.0005). Composite genotype or haplotype analyses of all 3 IL6 promoter variants showed associations with type 2 diabetes (p<0.002), obesity (p<0.02), and the metabolic syndrome (p<0.01).

CONCLUSIONS

The present studies suggest that single-nucleotide polymorphisms and composite genotypes or haplotypes of the IL6 promoter may be associated with several features of the metabolic syndrome in Caucasians.

Leptin: structure, function and biology

Leptin is an adipocyte-derived hormone that acts as a major regulator for food intake and energy homeostasis. Leptin deficiency or resistance can result in profound obesity, diabetes, and infertility in humans. Since its discovery, our understanding of leptin's biological functions has expanded from anti-obesity to broad effects on reproduction, hematopoiesis, angiogenesis, blood pressure, bone mass, lymphoid organ homeostasis, and T lymphocyte systems. Leptin orchestrates complex biological effects through its receptors, expressed both centrally and peripherally. Leptin receptor belongs to the class I cytokine receptor superfamily. At least five isoforms of leptin receptor exist, primarily because of alternate splicing. The longest form is capable of full signal transduction. The short forms may serve as leptin binding proteins and play a role in leptin transporting across the blood-brain barrier. In this review, we present the crystal structure of leptin and the structural comparison with other four-helical cytokines, discuss the leptin-receptor binding models based on other cytokine-receptor complex structures, and summarize the most recent progress on leptin signal transduction pathways--especially its link to peripheral lipid metabolism through AMP-activated protein kinase and hepatic stearoyl-CoA desaturase-1 pathways. Furthermore, we propose the structure based design of leptin analogs with increased stability, improved potency, enhanced blood-brain barrier transport, and extended time action for future therapeutic application.

Shared cytokine signaling receptors: structural insights from the gp130 system

The vast majority of cytokine signaling is mediated by "shared" receptors that form central signaling components of higher-order complexes incorporating ligand-specific receptors. These include the common gamma chain (gamma(c)), common beta chain (beta(c)), and gp130, as well as others. These receptors have the dual tasks of cross-reactive cytokine recognition, and formation of precisely oriented multimeric signaling assemblies. Currently, detailed structural information on a shared receptor complex exists only for gp130, which is a highly pleiotropic shared cytokine signaling receptor essential for mammalian cell growth and homeostasis. To date, more than 10 different four-helix bundle ligands have been identified that incorporate gp130, or one of its close relatives such as LIF receptor, into functional oligomeric signaling complexes. In this review we summarize our current knowledge of shared receptor recognition and activation, with a focus on gp130. We discuss recent structural and functional information to analyze overall architectural assemblies of gp130 cytokine complexes and probe the basis for the extreme cross-reactivity of gp130 for its multiple cytokine ligands.

A novel hIL-6 antagonist peptide from computer-aided design contributes to suppression of apoptosis in M1 cells

Based on the complex crystal structure of human interleukin-6 (hIL-6) and its receptor (hIL-6R), a novel hIL-6 antagonist peptide (named PT) was designed using computer-guided design method. Dealing with molecular docking and molecular dynamics methods, the interaction between PT and hIL-6R was analyzed. The theoretical studies showed that PT possessed very high affinity to hIL-6R and offered a practical means of imposing long-term blockade of hIL-6 activity in vivo. This effect was examined due to growth arrest and apoptosis induced by hIL-6 in myeloblastic cell line M1 cells in a dose-dependent manner. The findings demonstrate that PT could also act as an excellent antagonist candidate for the induction of growth arrest and apoptosis. Furthermore, murine M1 myeloid cell line, which was induced by the physiological inducer hIL-6 to undergo apoptosis and growth arrest, could be used as a subtle model system to test hIL-6 antagonist.

Kaposi's Sarcoma-associated Herpesvirus-encoded Viral Interleukin-6 Is Secreted and Modified Differently Than Human Interleukin-6

Viral interleukin-6 (vIL-6) is a homolog of cellular IL-6 that is encoded by the Kaposi's sarcoma-associated herpesvirus (KSHV) genome. vIL-6 binds to the IL-6 signal transducer gp130 without the cooperation of the IL-6 high affinity receptor to induce STAT3 DNA binding and cell proliferation. Although vIL-6 is believed to be important in the pathogenesis of KSHV-induced diseases, its secretion and post-translational modifications have not previously been characterized. Pulse-chase analysis revealed that the half-time of vIL-6 secretion is approximately 8-fold longer than that of human IL-6. Yet, the vIL-6 signal sequence targets human IL-6 secretion to nearly wild-type levels. Surprisingly, vIL-6 was not secreted from a cell line that does not express gp130 but expression of human gp130 in these cells enabled the secretion of vIL-6. Consistent with this observation, complete maturation of gp130 N-glycans is inhibited by vIL-6 coexpression, suggesting that the binding of the receptor to vIL-6 occurs intracellularly in early or pre-Golgi compartments. Furthermore, a vIL-6 mutant containing an endoplasmic reticulum retention signal is not secreted but does still induce receptor activation and signaling. Secreted vIL-6 is completely glycosylated at both possible N-glycosylaton sites and contains a large proportion of immature high-mannose glycans that is not typical of cytokines. These findings suggest that vIL-6 may induce gp130 signaling by an exclusively autocrine mechanism that relies on intracellular binding to its receptor. During KSHV infection, vIL-6 may only induce signaling in KSHV-infected cells to benefit the viral life cycle and promote oncogenic transformation.

Two Different Contact Sites Are Recruited by Cardiotrophin-like Cytokine (CLC) to Generate the CLC/CLF and CLC/sCNTFRα Composite Cytokines

The cytokines of the interleukin-6 family are multifunctional proteins that regulate cell growth, differentiation, and other cell functions in a variety of biological systems including the immune, inflammatory, hematopoietic, and nervous systems. One member of this family, ciliary neurotrophic factor (CNTF), displays biological functions more restricted to the neuromuscular axis. We have recently identified two additional ligands for the CNTF receptor complex. Both are composite cytokines formed by cardiotrophin-like cytokine (CLC) associated to either the soluble type I cytokine receptor CLF or the soluble form of CNTF receptor alpha (CNTFRalpha). The present study was aimed at analyzing the interactions between the cytokine CLC and its different receptor chains. For this purpose, we modeled CLC/receptor interactions to define the residues potentially involved in the contact sites. We then performed site-directed mutagenesis on these residues and analyzed the biological interactions between mutants and receptor chains. Importantly, we found that CLC interacts with the soluble forms of CNTFRalpha and CLF via sites 1 and 3, respectively. For site 1, the most crucial residues involved in the interaction are Trp67, Arg170, and Asp174, which interact with CNTFRalpha. Surprisingly, the residues that are important for the interaction of CLC with CLF are part of the conserved FXXK motif of site 3 known to be the interaction site of LIFRbeta. Obtained results show that the Phe151 and Lys154 residues are effectively involved in the interaction of CLC with LIFRbeta. This study establishes the molecular details of the interaction of CLC with CLF, CNTFRalpha, and LIFRbeta and helps to define the precise role of each protein in this functional receptor complex.

A flexible activin explains the membrane-dependent cooperative assembly of TGF-beta family receptors

A new crystal structure of activin in complex with the extracellular domain of its type II receptor (ActRIIb-ECD) shows that the ligand exhibits an unexpected flexibility. The motion in the activin dimer disrupts its type I receptor interface, which may account for the disparity in its affinity for type I versus type II receptors. We have measured the affinities of activin and its antagonist inhibin for ActRIIb-ECD and found that the affinity of the 2-fold symmetric homodimer activin for ActRIIb-ECD depends on the availability of two spatially coupled ActRIIb-ECD molecules, whereas the affinity of the heterodimer inhibin does not. Our results indicate that activin's affinity for its two receptor types is greatly influenced by their membrane-restricted setting. We propose that activin affinity is modulated by the ligand flexibility and that cooperativity is achieved by binding to two ActRII chains that immobilize activin in a type I binding-competent orientation.

The IL-10R2 binding hot spot on IL-22 is located on the N-terminal helix and is dependent on N-linked glycosylation

IL-22 is a class 2 alpha-helical cytokine involved in the generation of inflammatory responses. These activities require IL-22 to engage the cell surface receptors IL-22R1 and the low-affinity signaling molecule IL-10R2. IL-10R2 also interacts with five other class 2 cytokines: IL-10, IL-26, and the interferon-like cytokines IL-28A, IL-28B, and IL-29. Here, we define the IL-10R2 binding site on IL-22 using surface plasmon resonance (SPR) and site-directed mutagenesis. Surprisingly, the binding hot spot on IL-22 includes asparagine 54 (N54), which is post-translationally modified by N-linked glycosylation. Further characterization of the glycosylation reveals that only a single fucosylated N-acetyl glucosamine on N54 is required for maximal IL-10R2 binding. Biological responses of IL-22 mutants measured in cell-based luciferase assays correlate with the in vitro SPR studies. Together, these data suggest that IL-22 activity may be modulated via changes in the glycosylation state of the ligand during inflammation.

Noncovalent Multivalent Assembly of Jun Peptides on a Leucine Zipper Dendrimer Displaying Fos Peptides

[reaction: see text] The synthesis and characterization of a new leucine-zipper dendrimer (LZD) is reported that displays four copies of the peptide corresponding to the coiled-coiled dimerization domain of Fos. Circular dichroism spectroscopy, fluorescence titration, and sedimentation equilibrium experiments demonstrate that Fos-LZD can noncovalently assemble four copies of the peptide corresponding to the coiled-coil domain of Jun. This work provides the basis for the future construction of noncovalently assembled multivalent protein assemblies displaying any protein of interest.

Mapping of the Leptin Binding Sites and Design of a Leptin Antagonist

The leptin/leptin receptor system shows strong similarities to the long-chain cytokine interleukin-6 (IL-6) and granulocyte colony-stimulating factor cytokine/receptor systems. The IL-6 family cytokines interact with their receptors through three different binding sites I-III. The leptin structure was superposed on the crystal structures of several long-chain cytokines, and a series of leptin mutants was generated focusing on binding sites I-III. The effect of the mutations on leptin receptor (LR) signaling and on binding to the membrane proximal cytokine receptor homology domain (CRH2) of the LR was determined. Mutations in binding site I at the C terminus of helix D show a modest effect on signaling and do not affect binding to CRH2. Binding site II is composed of residues at the surface of helices A and C. Mutations in this site impair binding to CRH2 but have only limited effect on signaling. Site III mutations around the N terminus of helix D impair receptor activation without affecting binding to CRH2. We identified an S120A/T121A mutant in binding site III, which lacks any signaling capacity, but which still binds to CRH2 with wild type affinity. This leptin mutant behaves as a potent leptin antagonist both in vitro and in vivo.

Regulation of haematopoiesis by growth factors - emerging insights and therapies

Haematopoiesis is regulated by a wide variety of glycoprotein hormones, including stem cell factor, granulocyte-macrophage colony-stimulating factor, thrombopoietin and IL-3. These haematopoietic growth factors (HGFs) share a number of properties, including redundancy, pleiotropy, autocrine and paracrine effects, receptor subunit oligomerisation and similar signal transduction mechanisms, yet each one has a unique spectrum of haematopoietic activity. Ongoing studies with knockout mice have discovered previously unrecognised physiological roles for HGFs, linking haematopoiesis to innate immunity, pulmonary physiology and bone metabolism. The regulation of stem cells by HGFs within niches of the bone marrow microenvironment is now well recognised and similar mechanisms appear to exist in the regulation of other stem cell compartments. Alternative signalling strategies, other than tyrosine kinase activation and phosphotyrosine cascades, may account for some of the more subtle differences between HGFs. Accumulating evidence suggests that some, but not all, HGF receptors can transduce a genuine lineage-determining signal at certain points in haematopoiesis. Further studies, primarily at the receptor level, are needed to determine the mechanisms of instructive signalling, which may include phosphoserine cascades. Novel haematopoietic regulators, as well as the development of biological therapies, including growth factor antagonists and peptide mimetics, are also discussed.

Characterization of the Signaling Capacities of the Novel gp130-like Cytokine Receptor

The gp130-like receptor (GPL) is a recently cloned member of the family of type I cytokine receptors. The name reflects its close relationship to gp130, the common receptor subunit of the interleukin (IL)-6-type cytokines. Indeed, the recently proposed ligand for GPL, IL-31, is closely related to the IL-6-type cytokines oncostatin M, leukemia inhibitory factor, and cardiotrophin-1. The second signal transducing receptor for IL-31 seems to be the oncostatin M receptor beta (OSMRbeta). The present study characterizes in depth the molecular mechanisms underlying GPL-mediated signal transduction. GPL is a strong activator of STAT3 and STAT5, whereas STAT1 is only marginally tyrosine-phosphorylated. We identify tyrosine residues 652 and 721 in the cytoplasmic region of the longest isoform of GPL (GPL(745)) as the major STAT5- and STAT3-activating sites, respectively. Additionally, we demonstrate Jak1 binding to GPL and its activation in heteromeric complexes with the OSMRbeta but also in a homomeric receptor complex. Most interesting, unlike OSMRbeta and gp130, GPL is insufficient to mediate ERK1/2 phosphorylation. We propose that this is due to a lack of recruitment of the tyrosine phosphatase SHP-2 or the adaptor protein Shc to the cytoplasmic domain of GPL.

Compensatory energetic mechanisms mediating the assembly of signaling complexes between interleukin-2 and its alpha, beta, and gamma(c) receptors

Interleukin-2 is a key immuno-regulatory cytokine whose actions are mediated by three different cell surface receptors: the alpha, beta and the "common gamma" (gamma(c)) chains. We have undertaken a complete thermodynamic characterization of the stepwise assembly cycle for multiple possible combinations of the receptor-ligand, and receptor-receptor interactions that are necessary for formation of the high-affinity IL-2/alphabetagamma(c) signaling complex. We find an entropically favorable high affinity interaction between IL-2 and its alpha receptor, a moderately entropically favorable low affinity interaction between IL-2 and its beta receptor, and no interaction between IL-2 and the shared receptor, gamma(c). Formation of the stable intermediate trimolecular complexes of IL-2 with alpha and beta receptors, as well as IL-2 with beta and gamma(c) receptors proceeds through enthalpy-entropy compensation mechanisms. Surprisingly, we see a moderate affinity interaction between the unliganded receptor alpha and beta chains, suggesting that a preformed alphabeta complex may serve as the initial interaction complex for IL-2. Reconstitution of the IL-2/Ralphabetagamma(c) high-affinity quaternary signaling complex shows it to be assembled through cooperative energetics to form a 1:1:1:1 assembly. Collectively, the favorable entropy of the bimolecular interactions appears to be offset by the loss in rigid body entropy of the receptor components in the higher-order complexes, but overcome by the formation of increasingly enthalpically favorable composite interfaces. This enthalpic mechanism utilized by gamma(c) contrasts with the favorable entropic mechanism utilized by gp130 for degenerate cytokine interaction. In conclusion, we find that several energetically redundant pathways exist for formation of IL-2 receptor signaling complexes, suggesting a more complex equilibrium on the cell surface than has been previously appreciated.

The design of antagonist peptide of hIL-6 based on the binding epitope of hIL-6 by computer-aided molecular modeling

The interaction between human interleukin-6 (hIL-6) and human interleukin-6 receptor (hIL-6R) is the initial and most specific step in the hIL-6 signaling pathway. Understanding its binding core and interaction mechanism at amino acid level is the basis for designing small IL-6 inhibiting molecules, such as peptides or lead compounds. With Docking method, the complex structure composed of hIL-6 and its alpha-subunit receptor (hIL-6R) was analyzed theoretically. By using structure-based analysis and phage display methods, the loop AB (from Lys67 to Glu81) of hIL-6 was found to be the important binding epitope of hIL-6R. By means of computer-aided design, the mimic antagonist peptide (14 residues) was designed and synthesized. Using multiple myeloma cell line (XG7), IL-6 dependent cell line, as test model, the influence of antagonist peptides on the proliferation of XG7 cells was investigated. The results showed that the synthetic peptide could be competitive to bind to hIL-6R with hIL-6, and the effect was concentration dependent. The theoretical design approach is a powerful alternative to phage peptide library for protein mimics. Such mini-peptide is more amenable to synthetic chemistry and thus may be useful starting points for the design of small organic mimics.

The role of the inhibitors of interleukin-6 signal transduction SHP2 and SOCS3 for desensitization of interleukin-6 signalling

The immediate early response of cells treated with IL-6 (interleukin-6) is the activation of the signal transducer and activator of transcription (STAT)3. The Src homology domain 2 (SH2)-containing protein tyrosine phosphatase SHP2 and the feedback inhibitor SOCS3 (suppressor of cytokine signalling) are potent inhibitors of IL-6 signal transduction. Impaired function of SOCS3 or SHP2 leads to enhanced and prolonged IL-6 signalling. The inhibitory function of both proteins depends on their recruitment to the tyrosine motif 759 within glycoprotein gp130. In contrast to inactivation, desensitization of signal transduction is regarded as impaired responsiveness due to prestimulation. Usually, after activation the sensing receptor becomes inactivated by modifications such as phosphorylation, internalization or degradation. We designed an experimental approach which allows discrimination between desensitization and inactivation of IL-6 signal transduction. We observed that pre-stimulation with IL-6 renders cells less sensitive to further stimulation with IL-6. After several hours, the cells become sensitive again. We show that not only signal transduction through previously activated receptors is affected by desensitization but signalling through receptors which were not targeted by the first stimulation was also attenuated ( trans -desensitization). Interestingly, in contrast to inhibition, desensitization does not depend on the presence of functional SHP2. Furthermore, cells lacking SOCS3 show constitutive STAT3 activation which is not affected by pre-stimulation with IL-6. All these observations suggest that desensitization and inhibition of signalling are mechanistically distinct.

Uterine Msx-1 and Wnt4 Signaling Becomes Aberrant in Mice with the Loss of Leukemia Inhibitory Factor or Hoxa-10: Evidence for a Novel Cytokine-Homeobox-Wnt Signaling in Implantation

Successful implantation absolutely depends on the reciprocal interaction between the implantation-competent blastocyst and the receptive uterus. Expression and gene targeting studies have shown that leukemia inhibitory factor (LIF), a cytokine of the IL-6 family, and Hoxa-10, an abdominalB-like homeobox gene, are crucial to implantation and decidualization in mice. Using these mutant mice, we sought to determine the importance of Msx-1 (another homeobox gene formerly known as Hox-7.1) and of Wnt4 (a ligand of the Wnt family) signaling in implantation because of their reported functions during development. We observed that Msx-1, Wnt4, and a Wnt antagonist sFRP4 are differentially expressed in the mouse uterus during the periimplantation period, suggesting their role in implantation. In addition, we observed an aberrant uterine expression of Msx-1 and sFRP4 in Lif mutant mice, and of Wnt4 and sFRP4 in Hoxa-10 mutant mice, further reinforcing the importance of these signaling pathways in implantation. Collectively, the present results provide evidence for a novel cytokine-homeotic-Wnt signaling network in implantation.

Molecular recognition of BMP-2 and BMP receptor IA

Bone morphogenetic protein-2 (BMP-2) and other members of the TGF-beta superfamily regulate the development, maintenance and regeneration of tissues and organs. Binding epitopes for these extracellular signaling proteins have been defined, but hot spots specifying binding affinity and specificity have so far not been identified. In this study, mutational and structural analyses show that epitopes of BMP-2 and the BRIA receptor form a new type of protein-protein interface. The main chain atoms of Leu 51 and Asp53 of BMP-2 represent a hot spot of binding to BRIA. The BMP-2 variant L51P was deficient in type I receptor binding only, whereas its overall structure and its binding to type II receptors and modulator proteins, such as noggin, were unchanged. Thus, the L51P substitution converts BMP-2 into a receptor-inactive inhibitor of noggin. These results are relevant for other proteins of the TGF-beta superfamily and provide useful clues for structure-based drug design.

Molecular mechanisms for viral mimicry of a human cytokine: activation of gp130 by HHV-8 interleukin-6

Kaposi's sarcoma-associated herpesvirus (KSHV, or HHV-8) encodes a pathogenic viral homologue of human interleukin-6 (IL-6). In contrast to human IL-6 (hIL-6), viral IL-6 (vIL-6) binds directly to, and activates, the shared human cytokine signaling receptor gp130 without the requirement for pre-complexation to a specific alpha-receptor. Here, we dissect the biochemical and functional basis of vIL-6 mimicry of hIL-6. We find that, in addition to the "alpha-receptor-independent" tetrameric vIL-6/gp130 complex, the viral cytokine can engage the human alpha-receptor (IL-6Ralpha) to form a hexameric vIL-6/IL-6Ralpha/gp130 complex with enhanced signaling potency. In contrast to the assembly sequence of the hIL-6 hexamer, the preformed vIL-6/gp130 tetramer can be decorated with IL-6Ralpha, post facto, in a "vIL-6-dependent" fashion. A detailed comparison of the viral and human cytokine/gp130 interfaces indicates that vIL-6 has evolved a unique molecular strategy to interact with gp130, as revealed by an almost entirely divergent structural makeup of its receptor binding sites. Viral IL-6 appears to utilize an elegant combination of both convergent, and unexpectedly divergent, molecular strategies to oligomerize gp130 and activate similar downstream signaling cascades as its human counterpart.

The Class II cytokine receptor (CRF2) family: overview and patterns of receptor–ligand interactions

Expanded genomic information has driven the discovery of new members of the human Class II family of cytokine receptors (CRF2), which now includes 12 proteins. The corresponding cytokines have been identified, paired with their receptors and initially characterized for function. These cytokines include: a new human Type I IFN, IFN-kappa; molecules related to IL-10 (IL-19, IL-20, IL-22, IL-24, IL-26); and IFN-lambdas (IL-28/29), which have antiviral and cell stimulatory activities reminiscent of Type I IFNs, but act through a distinct receptor. In response to ligand binding, the CRF2 proteins form heterodimers, leading to cytokine-specific cellular responses; these diverse physiological functions are just beginning to be explored. Progress in structural and mutational analysis of ligand-receptor interactions now presents a more reliable framework for understanding receptor-ligand interactions, and for predicting key regions in less well studied members of the CRF2 family. The relationships between the CRF2 proteins will be summarized, as will the progress in identifying patterns of receptor interactions with ligands.

Direct Determination of the Interleukin-6 Binding Epitope of the Interleukin-6 Receptor by NMR Spectroscopy

All cytokines belonging to the interleukin-6 (IL-6)-type family of cytokines utilize receptors that have a modular build of several immunoglobulin-like and fibronectin type III-like domains. Characteristic of these receptors is a cytokine receptor homology region consisting of two such fibronectin domains defined by a set of four conserved cysteines and a tryptophan-serine-X-tryptophan-serine sequence motif. On target cells, interleukin-6 first binds to its specific receptor and subsequently to a homodimer of the signal transducer protein gp130. The interleukin-6 receptor consists of three extracellular domains. The N-terminal immunoglobulin-like domain is not involved in ligand binding, whereas the third membrane proximal fibronectin-like domain accounts for more than 90% of the binding energy to IL-6. Here, the key residues of this fibronectin-like domain involved in the interaction with IL-6 are described. Chemical shift mapping data with 15N-labeled IL-6R-D3 and unlabeled IL-6 coupled with recent structural data clearly reveal the epitope within the IL-6R-D3 responsible for mediating the high affinity interaction with its cognate cytokine.

Convergent mechanisms for recognition of divergent cytokines by the shared signaling receptor gp130

Gp130 is a shared cell-surface signaling receptor for at least ten different hematopoietic cytokines, but the basis of its degenerate recognition properties is unknown. We have determined the crystal structure of human leukemia inhibitory factor (LIF) bound to the cytokine binding region (CHR) of gp130 at 2.5 A resolution. Strikingly, we find that the shared binding site on gp130 has an entirely rigid core, while the LIF binding interface diverges sharply in structure and chemistry from that of other gp130 ligands. Dissection of the LIF-gp130 interface, along with comparative studies of other gp130 cytokines, reveal that gp130 has evolved a "thermodynamic plasticity" that is relatively insensitive to ligand structure, to enable crossreactivity. These observations reveal a novel and alternative mechanism for degenerate recognition from that of structural plasticity.